According to the prior art, a Bragg grating in an optical fiber is usually created in the core region of the optical fiber. A core region suitable for an optical fiber in which a Bragg grating is to be created typically includes a core region of silica containing germanium or other known photosensitizing dopants so as to impart to the core region, by exposure to ultraviolet (UV) light, the photo-refractive structure known as a Bragg grating.
Bragg gratings are generally produced in such a doped silica core of an optical fiber by laterally exposing the optical fiber to a three dimensional fringe pattern created by holographically interfering two coherent high intensity coherent UV beams, or by exposing the optical fiber to UV light passed through a diffractive optical element called a phase mask. The doped glass, by virtue of the lattice defects (in this case point imperfections) associated with the dopants, interacts with the bright portions of a UV pattern to produce light-absorbing color centers. Either technique produces a pattern of UV light consisting of alternating bright and dark regions. The doped glass interacts with the bright portions of the UV pattern in such a way that its refractive index is modified leaving a refractive index modulated according to the UV pattern.
The use of Bragg gratings has been demonstrated in commercial telecommunications-grade optical fibers that contain germanium in the core (Corning SMF-28 for example). However, high photosensitivity of these optical fibers is needed to achieve high reflectivity gratings and compatibility with manufacturing processes within reasonable exposure parameters. This has led to the use of hydrogenation, along with the development of special optical fibers containing high levels of germanium or other photosensitizing dopants, as a means to increase the number of defect centers to promote optical fiber photosensitivity. Pure silica core optical fibers, which contain little or no such defects, have been found unsuitable as a host material for forming Bragg gratings using UV exposure processes.
In step-index single-mode optical fibers, the transverse field distribution of the fundamental mode extends slightly beyond the core region, with a small amount of power carried in the part of the cladding immediately adjacent the core. The amount of power is on the order of 10-12% with mode parameters of typical communications-grade optical fibers.
In providing a Bragg grating in an optical fiber, coupling to cladding modes of light at the Bragg grating is sometimes suppressed by photosensitizing both the core and the cladding, and so extending the Bragg grating beyond the transverse mode field distribution. (See, e.g. E. Delevaque et al., "Optical Fiber Design For Strong Gratings Photoimprinting With Radiation Mode Suppression", Proc. Opt. Fiber. Comm. Conf., 1995, postdeadline paper PD5.) This approach demonstrates the ability to imprint a Bragg grating in a photosensitized cladding region of an optical fiber as well as in a photosensitized core. It also demonstrates light interaction between the mode evanescent field and fiber cladding region of the fiber.
With this approach, the reflectivity of the Bragg grating is high, even though only a small percentage of the reflected power is conveyed by the cladding. In a number of applications, however, what is needed is low reflectivity of light by a Bragg grating. Such applications include defining arrays of grating-based Fabry-Perot interferometers, and providing wavelength stabilization of laser diodes. If a Bragg grating could be formed in only the cladding of an optical fiber, the reflected power would be suitable for such applications.